Disagreement over the definition of atheist and agnostic has cluttered up various threads here, scattering confusion in its wake like a muckspreader in autumn.

The cause of the confusion is that atheists and theists have different definitions of the words agnostic and atheist, and adamantly refuse to accept the validity of each other's definitions.

Here is a short form of the definitions from the two separate points of view.

Theist version: An atheist is certain there is no God, an agnostic is not certain.

Atheist version: An atheist believes there is no God, an agnostic doesn't know.

The two versions are only subtly different, but a great deal of hot air has been expended on this difference.

Let's look at the two definitions of atheist first, because this is where the cause of the confusion really resides. It is the distinction between "believes" and "is certain". In choosing the two different forms of words, I am trying to convey that the theists' definition of atheism suggests that atheists know beyond any possibility of doubt that they have proof of God's nonexistence. The self-described atheists tend to use the word "believe" as meaning a very high degree of confidence, sufficient to live their lives on this basis, but falling short of 100% proven certainty.

When atheists try to explain this, a common riposte from theists is "You're not 100% certain, so you're not an atheist, you're just an agnostic, because you don't really know!"

There is a reason why some theists define atheism in these terms. If they define atheists as being 100% certain of the non-existence of God, then they can claim that atheists hold their view as a faith position. This appears to make some theists more comfortable, it frames the debate in more familiar terms – a religious battle between competing faiths. Also, by widening the definition of agnostic as far as possible, I suspect that some theists feel more comfortable with the idea that these waverers may in due course return to the one true faith.

There are very few self-described atheists who conform to the theists' definition of atheism. This is because the great majority of atheists have a scientific understanding of the world, and do not hold their atheism as a matter of faith, but rather through their understanding of the balance of evidence. They are aware that in principle some new piece of evidence might turn up tomorrow, and they leave themselves open to that possibility, no matter how unlikely they believe it to be.

If you accept that some degree of confidence short of 100% proved certainty of God's nonexistence is sufficient to allow somebody to qualify for the description of atheist, then how short of 100% certainty do you have to be before agnostic is a more accurate description? It's a fuzzy boundary and there is no objective answer. I take the practical view that if people are sufficiently certain they call themselves atheists and in practice live their lives on the assumption of God's nonexistence, then it is not very useful to insist on describing them as agnostic.

As for the boundary between believers in any faith and agnostics, I'm quite happy to apply the same principle. A Christian is somebody who says he is a Christian, and an agnostic is somebody who says he doesn't know.

If we all accept each other's self-applied labels, we can all get along much better.